The invention discloses a manufacturing method of a heat-resistant low-voltage electric wire applied to an ultra-thin-wall automobile. The method comprises the steps of sorting, drawing, copper wire annealing, stranding, insulation layer, insulation layer irradiation, spark rewinding, looping, packaging, qualification certificate sticking and warehousing. At the step of insulation, an extruder isused to extrude an insulating material out of a conductor. A derivometer and an infrared outer diameter detector are used to detect the eccentricity and outer diameter of the insulation layer. The infrared outer diameter detector is in butt joint with the extruder through an incremental PID control system. According to the invention, the incremental PID control system is used for butt joint; through outer diameter input of a target, synchronization of outer diameter control and screw speed can be quickly achieved, and unnecessary waste at the beginning is reducied; during high-speed production, the difference in product outer diameters is identified, and screw speed is adjusted in time; product outer diameters are controlled; and the produced cable has the advantages of small outer diameter, small insulation layer thickness, low eccentricity, even distribution, convenient heat dissipation, can be used in high temperature environments, and has small footprint when laid.